1. Field of the Invention
The present invention relates to an easy-opening can end used for beverage cans and food cans, which cans are opened by fracturing the opening section formed on the can end on the can, and relates to a making method thereof.
2. Description of the Related Arts
Easy-opening can ends are widely used as the lids of cans containing various kinds of drinks such as beer, juice, and coffee, to open the can by breaking the opening section formed on the can end by pressing the opening section with finger. Easy-opening can ends are roughly classified to the partial-open can ends used mainly in beverage cans, and the full-open can ends used mainly in food cans.
The partial-open can ends are further grouped into the pull-top tab can ends and the stay-on tab can ends. FIG. 8 shows a plan view of an example of the pull-top tab can ends. The procedure to open the pull-top tab can end illustrated in FIG. 8 is given below. That is, a tab 3 fixed to the center of a central panel section 8 made of a metal sheet such as steel or aluminum sheet as a part of a can end 1 using a rivet mechanism 9 is pulled up. With the resulted force of the lever rule, the working edge of the tab 3 pushes down a break-opening section 10 around which a score 2 for opening the can is formed on the central panel section 8. As a result, the score 2 for opening the can is broken. Further pull-up of the tab 3 results in separation of the broken opening section piece from the can end 1.
FIG. 9 shows a plan view of an example of the stay-on tab can ends. The procedure to open the stay-on tab can end illustrated in FIG. 9 is given below. That is, a tab 3 fixed to the center of a central panel section 8 structuring a can end 1 using a rivet mechanism 9 is pulled up. With the resulted force of the lever rule, the working edge of the tab 3 pushes down a break-opening section 10 around which a score 2 for opening the can is formed on the central panel section 8. As a result, the score 2 for opening the can is broken. Further pull-up of the tab 3 propagates the breaking, thus pushing a portion of the resulted broken opening section piece into the can while the piece is kept connected with the can end 1.
Since the full-open can end has a score for opening the can along the outer periphery of the can end, pull-up of the tab fixed to the panel at near outer periphery of the can end allows the opening section piece to separate from the can end, as in the case of pull-top can end.
As illustrated in the prior art illustration FIG. 10, the formation of a score for opening the can in an easy-opening can end in the prior art is performed by press-forming using a working tool 12 which has a knife-edge protrusion having a specified profile of the opening section and by applying a heavy load to form the score for opening the can with score depths of half or more of the thickness of the can end plate 13 from the upper surface of the can end, thus giving the score 11 in a V-shape cross section.
The pull-up force of the conventional tabs described above for opening the easy-opening cans needs a large power, and child or aged person cannot easily open the cans.
There have been proposed several means to solve the above-described problems which arose in forming a score for opening the can by pressing down the working tool having a knife-edge shape protrusion, in, for example, JP-B-55-10454 (the term “JP-B-” herein referred signifies the “examined” Japanese patent publication”), JP-B-3-71500, JP-B-3-71501. All of these proposals, however, failed to sufficiently reduce the pull-up force of the tab.
Furthermore, JP-B-3-5890, JP-A-62-235053 (the term “JP-A-” herein referred signifies the “unexamined Japanese patent publication”), and JP-A-2-179329 disclose means to combine the thickness-reduction working and the working from inside of the can end to reduce the pull-up force of tab. Even these means do not sufficiently reduce the pull-up force of tab.
JP-A-8-224626 discloses a means to form a score for opening the can with a different shape from V-shaped score by combining compression, tension, and shearing works. The means, however, does not use a wrinkle-prevention press-plate so that the material in the vicinity of the forming section is subjected to tensile deformation during punch-pressing step, which generates reduction in sheet thickness, ending in degraded rigidity, and failing to concentrate the deformation to the score portion during opening step, and failing to fully reduce the can-opening force.
Since the formation of a score for opening the can is conducted using a working tool and under a heavy load of press machine, a can end made of a steel sheet coated by resin layer on both sides thereof induces damage on the resin coating layers on both sides of the can end during the press-forming stage, thus degrading the corrosion resistance of the can. Accordingly, to prevent the degradation of corrosion resistance, repair coating is requested after the press-forming, which requires excess amount of man-hour and cost.
There has recently been introduced an aluminum that does not generate rust even when the resin coating layer is damaged. The use of aluminum, however, increases cost and raises a problem in resource recycling.
As a means to solve the above-described problems encountered during the formation of a score for opening the can on a can end made of surface-treated steel sheet coated by resin layer, a method to form a score for opening the can by composite extrusion process is disclosed in JP-A-6-115546, JP-A-6-115547, and JP-A-6-115548. According to the disclosure, the score for opening the can is formed by the composite extrusion process so that the resin coated layer is not damaged and that no repair coating is required. The disclosure, however, does not give detailed description on the working conditions of the composite extrusion and on the score shape, thus it is difficult to judge the reproducibility of the stable score for opening the can.
JP-A-8-99140 discloses a method of forming a score by hot-working between upper and lower dies having shoulder radius ranging from 0.1 to 1.0 mm to attain thickness at the thinnest portion to half or less of the original thickness. The use of dies having radius ranging from 0.1 to 1.0 mm is effective against the damage of resin coating layer. The can-opening force is determined by the absolute value of the thickness at the thinnest portion, so even the values of thickness less than half the original one do not necessarily give good can-openability.
Examined Japanese utility model publication No. 63-40439 discloses the formation of a concavity for finger-insertion beneath the finger-picking section of the tab and on the central panel section aiming to broaden the gap between the central panel section of the can end and the finger-picking section of the tab for assuring easy insertion of finger and easy holding of the finger-picking section. Unexamined Japanese utility model publication No. 5-40133 discloses a structure for easy insertion of finger into a gap between the central panel section of the can end and the finger-picking section of the tab and for easy holding of the finger-picking section. According to the disclosure, a tab is fixed by a rivet in a manner that the tab is allowed to rotate to move from a disabled-opening position with an off-set between the center axis of the tab and the center axis of the break-opening section to an enabled-opening position with coincident center axes therebetween, thus the finger-picking section of the tab is lifted by a tapered protrusion formed on the central panel section between the rivet and the finger-picking section of the tab during the movement of the tab from the disabled-opening position to the enabled-opening position.
According to the above-described can end, a formed concavity for finger insertion or a formed tapered protrusion assures easy insertion of finger into a gap between the central panel section of the can end and the finger-picking section of the tab and easy holding of the finger-picking section compared with the can end having no concavity or tapered protrusion. Nevertheless, the pull-up force for opening the can does not differ between these cases, so the reduction in can-opening force is not attained.
Aluminum alloys are used as can lids in recent years rather than steel sheets owing to the soft in rigidity and the favorable can-openability compared with the steel sheets. The use of aluminum alloys, however, is not preferable because they increase cost than in the case of steel sheets. In addition, when the can shell is made of a steel sheet and only the can lid is made of aluminum plate, galvanic cell is formed to enhance corrosion of can depending on the contents of the can, which may result in pin-hole occurrence in a short time.
Furthermore, for efficient implementation of resource recycling which is promoted from the point of global environment conservation, a single material can structure is preferred. In this respect, development of an easy-opening can made of steel sheet that assures good can-openability is wanted.
Responding to these problems, studies were conducted on the steel sheets for can lids to improve the can-openability of the steel sheet easy-opening can lids in terms of base material. For example, JP-A-62-142746 discloses technology to improve the can-openability by limiting the thickness, yield point, and tensile strength of steel sheet within a specific range. JP-B-4-14169 discloses technology of manufacturing method of can lid to improve the can-openability by limiting the composition, thickness, yield point, and tensile strength of steel sheet within a specific range. JP-A-62-142746 and JP-B-4-14169, however, do not consider the material quality change resulted from work-hardening occurred during the score-forming step. In addition, JP-B-3-57179 discloses technology to improve the can-openability by suppressing the increase in strength at the score-forming section and by decreasing the elongation at the section through the limitation of the composition and hardness (HR30T) of the steel sheet in a specified range. Reduction in elongation, however, raises a problem to make the rivet working during lid-manufacturing step difficult.